It is known that certain compounds having a skeletal structure of 1,1,2-triphenyl-1-butene with one substituted aminoalkoxy group substituted at the 4-position of one of the phenyl groups at position 1 are known to have nonsteroidal antiestrogen activity. Tamoxifen, which is a compound of the formula ##STR3## is a typical example and is a useful therapeutic agent for hormone-dependent mammary cancer (breast cancer) owing to its potent antiestrogen activity (British Pat. No. 1,013,907).
Later, 4-hydroxytamoxifen, which is one of the metabolites of tamoxifen and has the formula ##STR4## was synthesized as a tamoxifen-related compound and its pharmacological properties were investigated (British Patent Application laid-open under No. 2003855).
In an in vitro study, 4-hydroxytamoxifen showed more potent antitumor activity than tamoxifen against experimental hormone-dependent tumor cells and was expected to be developed as a therapeutic agent for breast cancer. In an in vivo study, however, it was confirmed that it is inferior in effectiveness to tamoxifen (J. Biol. Chem. 1981, 256, 859; Cancer Res. 1982, 42, 317; Eur. J. Cancer Clin. Oncology 1980, 16, 239).